Heretofore, as a process for preparing N-substituted-N'-nitroisothiourea derivatives and N-cyclic(methyl)-N'-nitroisothiourea derivatives, which are useful as intermediates for the preparation of guanidine derivatives, there has been known a nitration process using sulfuric acid-fuming nitric acid for isothioureas [see "Journal of American Chemical Society," Vol. 76, p.1877, (1954)]. According to this known process, however, the yield in the nitration of isothioureas which have alkyl substituents on the nitrogen is extremely low and so this process has not been satisfactory as a general synthesis process or an industrial manufacturing process.